The free, unintegrated proviral DNA molecule of MuLV has recently been isolated in this laboratory. Work is presently progressing on a map of the restriction enzyme fragments of this DNA. The proposed research is designed to utilize this map to investigate the following problems: 1. The areas of homology between Moloney leukemia virus and other murine leukemia viruses. 2. The topology of the integrated viral genome and the nature of the site in the cell DNA into which the viral genome is inserted. 3. A comparison of the genomes of sarcoma and leukemia virus. 4. Techniques for the eventual mapping of viral genes on the restriction enzyme fragment map.